Modern aircraft typically include a pressurized air system that delivers hot pressurized engine bleed air for various uses within the aircraft, for example for carrying out the pressurization and air conditioning of the aircraft cabin. This hot bleed air, which reaches temperatures up to 200° C., is conveyed from the aircraft engines to the end user devices, such as air conditioning packs, in a pipe or conduit system. In the event of a rupture or leak of this pipe system, the hot pressurized air flows out of the pipe system and can damage other components or systems of the aircraft arranged near the location of the pipe rupture or leak.
In order to avoid such a dangerous problem, aircraft have typically included a monitoring system that uses sensors to detect and recognize a faulty escape of hot air from the pipe system, and uses a computer to generate warnings and/or to switch off the pressurized air supply. In order to reduce the amount of time necessary for searching for a rupture or air leak during maintenance and repair work on the rather extensive pipe system, the known monitoring systems also determine and indicate the general area or locality of the leak or overheating condition.
The sensors of the conventional overheating monitoring systems are each embodied in the form of a coaxial conductor arrangement having a respective electrical plug on each of its two ends. More particularly, the sensor includes an outer conductor embodied as a thin pipe or tube, an inner conductor embodied as a wire or the like running along in the center of the outer conductor pipe or tube, and a special salt compound pressed or enclosed in the annular space between the two conductors. At normal acceptable surrounding ambient temperatures, this salt compound acts as an electrical insulator. On the other hand, when the salt compound is heated above a prescribed temperature, the salt compound becomes electrically conductive and thus forms a low-resistance electrical path or connection between the inner and outer conductors at this overheated location. The so-called trigger temperature at which the salt compound becomes electrically conductive can be adjusted or preselected based on the composition or mixture proportions of the salt compound. Each one of these sensors is laid out along the pipeline that is to be monitored, and then connected by its electrical plugs to a computer, which evaluates the electrical data provided by the respective sensor.
In the conventional system, the computer measures the resistance between the outer conductor and the inner conductor of a respective sensor at regular time intervals. In the event a sensor, or a portion of a sensor at a particular location, overheats as a result of an unacceptable temperature increase in the area surrounding this location, the salt compound in the sensor at this location becomes electrically conductive and forms a low resistance electrical path between the inner and outer conductors at this location. As a result, the computer recognizes the low resistance of the sensor as an overheating condition and can then responsively generate an alarm indication or a switching or control signal for switching off the affected system. Furthermore, the computer can determine the location of the overheating condition by determining the resistance ratio of the inner conductor from the overheating location respectively to the two ends of the sensor.
The above described conventional system suffers at least the following disadvantages. The sensors require a rather complicated and costly manufacturing process, and also require a defined selection and setting of the desired trigger temperature during the manufacturing process. Once a sensor is manufactured, its trigger temperature is fixed. The tolerance range for the so-called trigger point or trigger temperature is relatively large, which diminishes the precision of the operation of the sensor. Since the sensors are designed and manufactured for a fixed temperature range, which is especially a rather narrow range, it becomes necessary to completely remove and exchange the sensors with different sensors if an effective temperature range of the monitoring system is to be changed. The ability to localize the area of the overheating condition using the conventional sensors in the conventional system is relatively inexact. Furthermore, the measurement signal given off by the sensor and received and evaluated by the computer must be shielded, because it is sensitive to electromagnetic interferences.